2 km... Since 1.8 is 2(.9), you do the same for the base of the smaller triangle (just multiply by two)
You can use this to help you. label the triangle like the picture and use the equations. I hope this helps.
Use the formula: A=2(3.14)rh+2(3.14)r^2
A=2(3.14)(6)(7)+2(3.14)6^2
A=490.09 so the answer rounded would be 490.1